Wavelength scale topography (grooves, holes, bumps, etc.) in metal films provide a means to couple free space photons to surface plasmons (electron oscillations combined with a surface electromagnetic wave). These structures have been shown to enhance transmission through sub-wavelength apertures at resonant wavelengths.
FIG. 1 shows a longitudinal cross-sectional view of a plasmon enhanced near-field optical probe 10(1). Certain features of FIG. 1 are exaggerated for clarity and are not drawn to scale. Plasmon enhanced near-field optical probe 10(1) has an optical fiber 20 that is, for example, a multimode ultraviolet (“UV”) grade fiber. Optical fiber 20 includes a full thickness region 26, where cladding 24 surrounds a core 22, and a tapered region 28, where cladding 24 thins and disappears (and where core 22 tapers, as shown). The side of fiber 20 in tapered region 28 is shown as side surface 42. Core 22 ends at fiber end surface 36.
Metal 30 coats side surface 42 and fiber end surface 36, except at an aperture 38. An outside surface of metal 30 in tapered region 28 is side surface 44, as shown. An outside surface of metal 30 counter-faces fiber end surface 36 at a metal end surface 32 as shown. The width of metal end surface 32 (shown by arrow 46) is for example about 5 microns.
Fiber end surface 36 and/or metal end surface 32 may be ruled. As shown in FIG. 1, both surfaces 32, 36 are ruled with rulings 31 having similar periodicity; although the periodicity of either surface may be adjusted to modify the performance of optical probe 10(1).
When electromagnetic (EM) radiation 40 (e.g., “EM radiation”) enters core 22, and enters tapered region 28, some of the EM radiation 40 exits core 22 at aperture 38. At fiber end surface 36, a surface plasmon may exist within metal 30, to interact with EM radiation 40 and increase the transmission of EM radiation 40 through aperture 38 and above the transmission obtainable in the absence of ruled surface 36. A surface plasmon may also exist within metal 30 at metal end surface 32, to interact with EM radiation 40 and alter its directionality when exiting aperture 38. The ruled periodicities of fiber end surface 36 and metal end surface 32 may vary to (a) enhance the transmission of EM radiation 40 through aperture 38 and (b) alter the directionality of EM radiation 40 exiting aperture 38.
The combination of a fiber end surface and a metal end surface, with at least one of the surfaces being ruled, is sometimes denoted herein a “plasmon enhancement structure.” In FIG. 1, fiber end surface 36 and metal end surface form plasmon enhancement structure 11(1). The combination of a plasmon enhancement structure (e.g., plasmon enhancement structure 11(1)) with an aperture (e.g., aperture 38) forms a “plasmon transmission structure.” The use of the terms “plasmon enhancement”, “plasmon transmission” and the like may encompass enhanced transmission and/or altered directionality of EM radiation passing through an aperture in a ruled surface, recognizing that underlying physical principles may be described in different terms (e.g., “coherent scattering”, “surface waves”, “coherent optical phenomenon” and the like).
An extension 34(1) attaches to metal 30 at metal end surface 32 to surround aperture 38 and extend outward (i.e., in the direction of arrow 12) from metal end surface 32.
Rulings 31 on the input surface of plasmon enhancement structure 11(1) have been shown to enhance the transmission through aperture 38 at resonant wavelengths, and rulings on the output surface 32 have been shown to focus or ‘beam’ the light transmitted through the aperture.